(a) Field of the Invention
The present invention relates to a fluid flow velocity measuring system, and in particular, to a fluid flow velocity measuring system using a crossed-beam laser Doppler velocimeter for measuring a flow velocity at a blind spot inside a combustion chamber.
(b) Description of the Related Art
A laser Doppler velocimeter (LDV) is used to measure a fluid flow, either liquid or gas. The LDV is employed to detect flow velocity in an internal combustion engine as a result of the LDV""s ability to measure the velocity in a flow field without affecting or disturbing the flow field.
FIG. 1 schematically shows a crossed-beam LDV system adapted to an internal combustion engine for detecting flow velocity in an exhaust manifold.
AN LDV system includes a laser 11 for emitting a laser beam (L) which passes through a transmitter 12. The transmitter 12 splits the beam (L) into two coherent beams having different frequencies and sends the two beams to a probe 14 via a dual-mode fiber optic 13. The probe 14 projects the two coherent laser beams, and the laser beams are caused to intersect at a predetermined point in a flow field of an exhaust manifold 20. The two beams produce a fringe pattern at this point of intersection. Particles 15 passing through this fringe pattern produce periodic variations in the amount of light scattered as the particles 15 pass through the light and dark crests of the fringe pattern. The scattered light from the fringe pattern is collected by photosensitive means (not shown) and the photosensitive means then generates electrical signals representing fluid velocity components.
Accordingly, the LDV provides a means of measuring fluid velocity without requiring the insertion of probes into the flow field. However, this prior art LDV system can not measure a fluid flow velocity around a valve seat in the combustion chamber because it is impossible to project a laser beam into the combustion chamber from the outside.
The present invention has been made in an effort to solve the above problems of the prior art.
It is an object of the present invention to provide to a fluid flow velocity measuring system using a crossed-beam laser Doppler velocimeter capable of measuring a flow velocity at a blind spot inside a combustion chamber, particularly around a valve seat. To achieve the above object, a fluid flow velocity measuring system for an internal combustion engine comprises a laser Doppler velocimeter having a probe for projecting a pair of coherent laser beams and beam guiding means for guiding the laser beams into a combustion chamber, the beam guiding means being installed in a spark plug mounting hole of an engine.